A magnetic recording medium is widely used as a tape for recording as a video tape or as a floppy disk. The magnetic recording medium is fundamentally composed of a nonmagnetic support having laminated thereon a magnetic layer containing ferromagnetic particles dispersed in a binder.
It is necessary that a magnetic recording medium has high levels of various characteristics such as electromagnetic characteristics, running durability or running performances. That is, higher performance for reproducing original sounds is required for an audio tape of recording and reproducing music. Excellent electromagnetic characteristics such as excellent performances for reproducing original images are required for a video tape.
It has been determined that frequency characteristics over the low sound range to high sound range of audio signals (D.C. 20 KHz or less) are improved by constructing plural (upper and under layers) magnetic layers in an audio tape to secure excellent electromagnetic characteristics. Such methods are disclosed in JP-A-58-17539, JP-A-58-56228 and JP-A-58-56229 (the term "JP-A" as used herein refers to a "published unexamined Japanese patent application").
In accordance with the above method, a magnetic recording medium wherein bias noise is decreased by distributing ferromagnetic particles having a large specific surface area on a surface of a magnetic layer and reproduced outputs are increased over all bandwidths by distributing ferromagnetic particles having a small specific surface area in the inside of the magnetic layer is disclosed in the above-cited Japanese references.
When plural layers are provided, that is, when an under magnetic layer and an upper magnetic layer are provided, first, an under magnetic layer is provided, dried and hardened, and thereafter, an upper layer is provided. The thickness of these magnetic layers, particularly that of the upper layer, which is thinner than that of the under layer, exceeds 1 .mu.m in almost all cases, and satisfactory smooth properties have been attained by the above method. The above method is employed upon preparing a linear audio (i.e., an audio having a fixed head system due to an alternating current bias method, which is used in an audio tape recorder which is generally used) type magnetic recording medium and the frequency to be used therein is in a low frequency range and is at most 20 KHz.
When signals having a wide bandwidth of frequency like video signals from a low frequency (linear video: 20 KHz or less), color signals (600 KHz to 800 KHz) to a high frequency (black-and-white signals: 8 MHz or less) are recorded and reproduced, frequency characteristics and noises such as noise occurring due to granular particles or tape modulation noise occurring due to surface roughness, etc., of a tape become significant problems.
Such problems regarding video tapes cannot be resolved satisfactorily by using the above method which is applied for a linear audio type magnetic recording medium.